I don't believe that cats feel jealousy and take it out on what they are jealous over, I just don't think that is in their psychological makeup. They are not humans.
What blood work and other testing did your vet do? Can you post the results? So your cat is 3 years old and he is not neutered? And they sort of got along in the beginning? Was Micaud a kitten when they were introduced? Has Spin played too rough with Micaud? Chased him?
Hopefully the neutering will help out
, hormones can cause many problems as they can in humans. In the meantime, perhaps some Feliway can help calm Micaud
. As 14+ stated, it can take some time to for the hormones to be out of his system.